Question

why do alpha particles have a high ionising power?

Answer 1

One alpha particle can ionise 10,000 atoms. However, because it puts all its energy intoionising others, it very quickly runs out of energy itself. Hence alpha particles can't penetrate through much. The fact that they are strongly ionising makes them very dangerous to life, however.

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Answer 2

Alpha particle has higher mass due to which its interaction with the matter is greater. Due to this greater collision rate, it looses energy to the atom with which it interacts and ionise it. This is the reason why it has more ionising power than the other radiations like beta and gamma which interacts lesser with matter as compare to alpha particle.


or

Ionisation is a 'collision' process between radiation particles and atoms .... with energy being transferred from the α / β particles at each interaction.

An α particle, having a mass more than 7000 times that of β, generally carries far more initial KE - enabling it to have more ionisation interactions before it's energy is depleted and it comes to rest.

Also, because of the α's greater size it's collision rate with atoms is greater than with β .. and -β's are readily deflected by an atom's repelling electrons so many collisions are avoided .. reducing the collision rate and giving weaker ionisation over a much longer path than for α.

The higher collision rate of alpha produces a greater concentration of ionisation